The present invention relates to electronic circuits that require a capacitor with a defined equivalent series resistance. In particular, the present invention relates to a method and apparatus that provides for an increase in the minimum total series resistance of a capacitor that may be used for compensation in a low-dropout regulator.
Voltage regulators are used to provide a relatively constant voltage source to other electronic circuits that are characterized as a xe2x80x9cloadxe2x80x9d. Some regulators have limited effectiveness in particular applications that require load regulation. For example, some regulators have a high xe2x80x9cdrop-outxe2x80x9d voltage. A xe2x80x9cdrop-outxe2x80x9d voltage is the minimum voltage difference between the input voltage and the output voltage that is necessary to maintain proper regulation. Large dropout voltages result in wasted power, and raise the minimum power supply requirements for maintaining proper regulation.
A low-dropout regulator (hereinafter referred to as an xe2x80x9cLDO regulatorxe2x80x9d) is useful in applications where it is desired to maintain a regulated voltage that is sufficiently close to the input voltage. LDO regulators are particularly useful in battery-powered applications where the power supply voltage is low. A capacitor may be added to the LDO regulator""s output for stability reasons.
The present invention is directed to a method and an apparatus for increasing a total effective resistance of a capacitor that may be used in an LDO regulator or other circuit that requires compensation. An improved LDO regulator includes an xe2x80x9con-chipxe2x80x9d resistor that is connected in series with the compensation capacitor to increase the total effective series resistance (ESR) of the capacitor. The xe2x80x9con-chipxe2x80x9d resistor may be an on-die resistor, an on-die metalization, a lead-frame, a bond-wire, a thin-film resistor, as well as others as applicable. An external capacitor is used to stabilize the LDO regulator. The stability of the LDO regulator is improved by placing the xe2x80x9con-chipxe2x80x9d resistor in series with the external capacitor to increase the total effective series resistance.
Briefly stated, voltage regulators use capacitors to compensate the voltage regulator and provide stable performance. Capacitors have an inherent equivalent series resistance (ESR) that changes over various operating conditions including signal frequency, operating temperature as well as others. An apparatus and method compensates for the low ESR of capacitors to increase the total equivalent series resistance of the capacitor. By providing an xe2x80x9con-chipxe2x80x9d resistance between the capacitor and the circuit ground potential, minimum total ESR can be provided such that stable load regulation is achieved with capacitors that would otherwise be undesirable for such use. Increasing the value of the output capacitor""s equivalent series resistance allows wider ranging values of capacitance to be used. The increased range of capacitance values allows capacitors of different material types to be used. Additionally, other arrangements with multiple resistors that are arranged as series resistors between the compensation capacitor and one or more of the power supplies provide stable load regulation.
In accordance with an example of the present invention, an apparatus for improving compensation in a regulator includes an external capacitor having an equivalent series resistance that is in a range of values. The capacitor is coupled to an output of the regulator. The apparatus also includes an on-chip resistor that has a value. The on-chip resistor is series coupled to the capacitor such that a total effective series resistance of the capacitor is increased. The total effective series resistance is the sum of the value and the range of values. The regulator is stabilized by the capacitor and the on-chip resistor.
In accordance with another example of the present invention, a method of compensating a regulator includes coupling a capacitor between a first on-chip terminal and a second on-chip terminal. The capacitor has an effective series resistance that is below a nominal value. The method also includes coupling one of the first on-chip terminal and the second on-chip terminal through an on-chip resistor to a potential. The sum of the values of the on-chip resistor and the effective series resistance are above the nominal value. The regulator is stabilized by the series combination of the on-chip resistor and the coupling capacitor.
In accordance with yet another example of the present invention, an apparatus for compensating a low drop-out regulator includes a capacitor that is coupled between an output of the low drop-out regulator and a circuit ground potential. The capacitor has an equivalent series resistance that is below a nominal value. The apparatus also includes a means for providing an on-chip resistance that is coupled to the capacitor. The sum of the values of the on-chip resistance and the equivalent series resistance are above the nominal value. The low drop-out regulator includes the on-chip resistor such that the series combination of the on-chip resistor and the capacitor enable stable operation of the low drop-out regulator.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.